It is known that petroleum resin and rosin metal salts are employed as resins for rotogravure inks and that petroleum resin metal salts are employed as resins for thermally fused road markings. They may also be used as resins for other paints and adhesives (Japanese Patent Publication Nos. 42-7,334 and 57-28,431).
The reaction of maleic anhydride-added petroleum resins with a divalent metal does not usually proceed in a state in which an oxide or a hydroxide of the metal is mixed with the maleated petroleum resin in a molten state at about 250.degree. C. In this case, the presence of an acetate of the metal in the system can make the reaction proceed. This reaction, however, presents the disadvantage that an organic acid is rendered free. It is also known that rosin can be linkewise reacted with a hydroxide of a metal at about 250.degree. C. to yield the corresponding rosin metal salt. However, these reactions, in which high temperatures are applied, present the drawbacks that thermal deterioration of the resin occurs, leading to a considerable debasement in resin hue. When such resins are used as rotogravure inks, the disadvantages are presented that the color tone is degraded and, when they are used for white paints, the paints will be tinged with yellow.
As a result of research on processes that may produce resin metal salts without causing any deterioration in the resin hue, it has been found that resin metal salts may be produced smoothly by reacting a resin in solution in an appropriate solvent with a hydroxide or oxide of a metal in the presence of a small amount of ammonia or an amine at low temperatures so that no thermal deterioration of the resin occurs. The present invention is based on this finding.